1. Field of the Invention
The present invention relates to a selected-wavelength tuning filter which is able to select an optical signal of desirable wavelength even when temperature changes or deterioration due to age occur and an optical add/drop multiplexer using the selected-wavelength tuning filter in an optical communication equipment for use in the optical communication network.
2. Description of the Related Art
Ultra-long-distance and large-capacity optical communication apparatuses are now required to construct future multimedia networks. Concentrated studies are now being made of the wavelength-division multiplexing as a method for realizing large-capacity apparatuses in view of such advantages that it can effectively utilize a wide bandwidth and a large capacity of an optical fiber.
Specifically, an optical communication network requires a function for passing, dropping and adding optical signals at each point on the network as necessary and an optical routing function and a cross-connecting function for selecting an optical transmission route. For this, researches and developments are being conducted on an optical add/drop multiplexer (hereinafter abbreviated as “OADM”) for passing, dropping and adding optical signals.
The OADM includes a wavelength-fixed type OADM that is able to drop/add only optical signal(s) of fixed wavelength and an arbitrary-wavelength type OADM that is able to drop/add optical signal(s) of arbitrary wavelength(s).
On the other hand, an acousto-optic selected-wavelength tuning filter (hereinafter abbreviated as “AOTF”) operates so as to extract only light of selected wavelength, so that its wavelength characteristic to an optical signal passing through the AOTF is flat. It is also able to select any wavelength, in contrast to an optical fiber grating, which is able to only select fixed wavelength. Still more, because it is a selected-wavelength tuning filter, it may be used also as a wavelength selecting filter in a tributary office, which is an office for dropping/adding optical signals between end offices.
The researches and developments have been conducted on the OADM using the AOTF from such reasons.
FIG. 20 is a block diagram showing the structure of the conventional AOTF.
In the figure, the AOTF comprises two optical waveguides 201 and 202 on a substrate of lithium niobate (LiNbO3) which presents a piezoelectric effect. These optical waveguides 201 and 202 cross each other at two points and polarization beam splitters (hereinafter abbreviated as “PBS”) 203 and 204 are provided at the parts where the two optical waveguides cross each other.
A SAW guide 206 made of metallic film is formed on the two optical waveguides 201 and 202 between the crossing parts. Surface acoustic wave is generated when a radio-frequency signal is applied to an inter-digital transducer (hereinafter abbreviated as “IDT”) 205 propagates through the SAW guide 206.
While light entered to this AOTF is light in which TE and TM mode lights are mixed, but it is separated by the PBS 203 into the TE and TM mode lights, which propagate through the optical waveguides 201 and 202. Here, when surface acoustic wave propagates along the SAW guide 206 by a radio-frequency signal of specific frequency being applied, refractive indices of the optical waveguides 201 and 202 change periodically at the parts where the optical waveguides 201 and 202 cross with the SAW guide 206. Therefore, the TE mode and the TM mode of only light of the wavelength that interacts with the periodic change of the refractive index among the entered lights are switched. Then, the PBS 204 changes the propagating direction of the switched lights. Only light of the interacting wavelength is selected as light to be dropped and light of wavelength which has not interacted is trajected and transmitted as output light.
On the other hand, in the same manner, added light is also separated by the PBS 203 into TE and TM mode lights, propagates in the optical waveguides 201 and 202, and interacts with the surface acoustic wave. Then, only the respective modes of the light having specific wavelengths which are switched, changing propagating directions, and then added, becoming output light.
Thus, because the AOTF can select and drop only the light having the wavelength corresponding to the frequency of the radio-frequency signal and can change wavelength of light to be selected by changing the frequency of the radio-frequency signal, it functions as a selected-wavelength tuning filter.
It is noted that in the present specification, the main part which is formed on the substrate of lithium niobate (LiNbO3) and where light is dropped/added (passed) shown in FIG. 20 will be called an AOTF or an AOTF section, and the whole device plus peripheral devices for dropping/adding (passing) light will be called a selected-wavelength tuning filter.
Further, because the AOTF can drop/add light of any wavelength, it may be used for the OADM. The applicant of the present invention has already filed the invention in which the AOTF is used for the OADM as Japanese Patent Application No. 090383/1998.
Next, the OADM described in Japanese Patent Application No. 090383/1998 which is not yet laid open will be explained.
FIG. 21 is a block diagram showing the first basic structure of the OADM using the AOTF as shown in FIG. 20. The OADM shown in FIG. 21 can drop and receive optical signals of eight wavelengths and can generate and add optical signals of eight wavelengths. Here, because the respective structures for receiving and processing the optical signals are the same, only one structure is shown and the others are omitted in the figure. The respective structures for generating the optical signals are also the same, so that only one structure is shown and the others are omitted in the figure.
In FIG. 21, a WDM optical signal is entered to the AOTF section 210 and an optical signal having a wavelength corresponding to the frequency of the radio-frequency signal applied to the AOTF section 210 is dropped as a branched optical signal from a branching port of the AOTF section 210. This branched optical signal is amplified by an optical amplifier 217 for amplifying light and is then entered to a 1×8 optical coupler 218. The branched optical signal is divided and branched into eight lights by the 1×8 optical coupler 218 and is entered to an AOTF section 219. Accordingly, the optical signals of all wavelengths branched by the AOTF section 210 are contained in each of the divided and branched optical signals. Therefore, the AOTF section 219 selects only the optical signal of a wavelength to be received and processed by an optical receiver 220, and then the optical receiver 220 receives and processes it.
On the other hand, an optical signal to be added is generated as follows.
A laser diode (hereinafter abbreviated as “LD”) 211, i.e., a light source, emits laser beams having wavelengths corresponding to the wavelengths of the optical signals to be added. LDs 211 are prepared in the number of optical signals to be added, and in the case in FIG. 21, there are eight. The laser beams from the eight LDs are entered to an 8×8 optical coupler 212. The 8×8 optical coupler 212 wavelength-multiplexes the lights of eight wavelengths and branches the wavelength-multiplexed light by dividing them into eight. The branched light is amplified by an optical amplifier 213 and then entered to an AOTF section 214. The AOTF section 214 selects and exits a light having a wavelength to be used for an addition optical signal among lights in which lights of eight wavelengths are multiplexed. The light selected by the AOTF section 214 is modulated by an optical modulator 215 and entered to an 8×1 optical multiplexer 216 as an optical signal. The 8×1 optical multiplexer 216 multiplexes the optical signals of the respective wavelengths to generate an addition optical signal. The generated addition optical signal is added to an adding port of the AOTF section 210.
Because the AOTF section 210 not only branches the optical signal of the desirable wavelength but also adds the optical signal having the same wavelength with the branched wavelength as described above, the added optical signal is added by the AOTF section 210, and is exited as a WDM optical signal from an output port of the AOTF section 210 together with the WDM optical signal which is not branched and passes through as it is.
Thus, the AOTF may be used for the part for passing, dropping and adding the WDM optical signal, the part for generating the addition optical signal and the part for receiving and processing the drop optical signal of the OADM.
FIG. 22 is a block diagram showing the second basic structure of the OADM using the AOTF sections shown in FIG. 20. The figure shows the OADM that can drop and receive optical signals of eight wavelengths and can generate and add optical signals of eight wavelengths. Here, the respective structures for receiving and processing the optical signal are the same, so that only one structure is shown and the other remaining structures are omitted in the figure. The respective structures for generating the optical signal are also the same, so that only one structure is shown and the other remaining structures are omitted in the figure. Still more, the same components with those in FIG. 21 are denoted by the same reference numerals and the explanation thereof will be omitted here.
In FIG. 22, a WDM optical signal is entered to an optical coupler 230 and branched into two signals. One branched WDM optical signal is entered to an AOTF section 231 and the other is entered to an optical amplifier 217. The other WDM optical signal is amplified by the optical amplifier 217 and entered to the 1×8 optical coupler 218. It is divided and branched into eight signals by the 1×8 optical coupler 218 and is entered to an AOTF section 219. The AOTF section 219 selects only an optical signal having a wavelength to be received and processed by the optical receiver 220 and then the optical receiver 220 receives and processes it.
On the other hand, the WDM optical signal entered to the AOTF section 231 selects an optical signal which is the same with that of the AOTF section 219 in the receiving processing part and selects an optical signal of an even (odd) number of channel of the WDM optical signal and exits it to a selection port which is not connected anywhere. Which means, the optical signal of the wavelength selected by the AOTF section 231 is abandoned. The WDM optical signal, which has passed through the AOTF section 231, is entered to an AOTF section 232. The AOTF section 232 also selects an optical signal which is the same with that of the AOTF section 219 in the receiving process and an optical signal of an odd (even) number of channel of the WDM optical signal and exits it to a selection port not connected anywhere. Then, the WDM optical signal, which has passed through the AOTF section 232, is entered to an optical coupler 233.
Here, the AOTF sections 231 and 232 are connected in tandem because the range of the wavelength selecting characteristic of the AOTF is wide and crosstalk occurs when neighboring optical signals having a wavelength at intervals of 0.8 nm prescribed in ITU-T G. Recommendation 692 are to be branched by one AOTF. For this reason, the crosstalk could be reduced to a level where the optical signal is receivable by making the WDM signal select the even (odd) numbered optical signals of the WDM optical signals in the first stage of the AOTF section 231 and by making the WDM signal select the even (odd) numbered optical signals of the WDM optical signals in the second stage of the AOTF section 232.
The optical signal to be added is generated in the same manner with the case shown in FIG. 21, so that the explanation thereof will be omitted here. The generated optical signal to be added is entered to an optical coupler 233, is multiplexed with the WDM optical signal which has passed through the AOTF sections 231 and 232 and is exited to the optical transmission line as a WDM optical signal.
Thus, the AOTFs are used for the part for passing, dropping and adding the WDM optical signal, the part for generating the optical signal to be added and the part for receiving and processing the branched optical signal in the OADM.
By the way, although the AOTF can select and drop only light having the wavelength corresponding to the frequency of the radio-frequency signal as described above, its temperature dependency with respect to selected wavelength is high. More specifically, when temperature rises by 1° C. when the radio-frequency signals of the same frequency are applied, the selected wavelength changes by 0.8 nm (100 GHz).
Therefore, in the OADM using the AOTFs, an optical signal of the next wavelength is selected instead of an optical signal of the wavelength to be selected in the WDM optical signal in which wavelengths are disposed at intervals of 0.8 nm. Moreover, because it is difficult to control the AOTFs used in the OADM all to the same temperature, it is difficult to select optical signals of the same wavelength even when radio-frequency signals of the same frequency are applied to all AOTFs to select the same wavelength.
Further, the wavelength selected by the radio-frequency signal is also sensitive to variation of devices in the production of the AOTFs and to deterioration due to age.
Because the selection of the wavelength by the AOTFs is controlled by separating a light into the TE and TM mode lights and by causing them to interact with the surface acoustic wave as described above, the wavelength to be selected is changed when the state of polarized wave changes.
The intensity of light to be selected of the AOTF also changes depending on input intensity of the radio-frequency signal to be applied. It means that the intensity of light exited to the selection port by the AOTF changes in the OADM as shown in FIG. 22. In case when the input intensity of the radio-frequency signal is not appropriate, it is difficult to fully reject an optical signal to be rejected by the AOTF when rejecting it by exiting to the selection port because not enough optical signals are selected to the selection port.